1. Field of the Invention
The present invention is directed to a thermal switch. More particularly, the present invention is directed to a device for alternately switching, on command, between having a high thermal conductivity and a low thermal conductivity.
2. Background Information
A thermal switch is a device for selectively conducting heat energy. In its on state, a thermal switch readily conducts heat energy. When switched into an off state, the thermal switch is a very poor conductor of heat energy. An ideal thermal switch has an infinite thermal conductivity in the ON state and a zero conductivity in the OFF state.
In cryogenic applications (e.g., below 60K), a suitable thermal switch does not exist in the prior art. An acceptable on-off ratio (ratio of ON state thermal conductance to OFF state thermal conductance) for cryogenic applications would be 1000:1. No prior art cryogenic thermal switch meets this criterion.
Referring to FIG. 1, a gas gap thermal switch is illustrated in cross-section. In this thermal switch, the hot side element 101 is in thermal contact with a heat source (not shown). The hot side element 101 is separated from cold side element 103 by a gap 105. The gap 105 is the space formed between the frustoconical member 106 projecting from the hot side element 101 and the frustoconical cavity 104 in the cold side element 103. A hermetically sealed container 107 surrounds the gap 105 and is used to selectively contain a thermally conductive gas.
When it is desired for the gas gap thermal switch to conduct in the ON state, the thermally conductive gas is placed inside 109 the hermetically sealed container 107 via the gas supply shown. When the thermally conductive gas is placed inside the hermetically sealed container 107, heat energy is allowed to migrate from the hot side element 101, across the gap 105 by means of the thermally conductive gas, to the cold side element 103. When it is desired to switch the gas gap thermal switch to the OFF state, the thermally conductive gas is evacuated from the hermetically sealed chamber 107, leaving a vacuum inside the void 109 and the gap 105. When chamber 107 is evacuated, only a very reduced amount of heat energy is transmitted from the hot side 101 to the cold side 103 and conducted away by the cryocooler (not shown).
The gas gap thermal switch of the prior art has two major disadvantages. One is that it is a very complex device (because of the necessary gas handling structures and control system therefor, not shown) and, thus, is prone to failure. It requires a perfect hermetic seal in which the thermally conductive gas is contained. If the seal fails, then the switch will not properly work in an ON condition. Another disadvantage is the critical alignment required that cannot be verified after integration.
A failed attempt has been made to construct a thermal switch based on the principle of differential coefficients of thermal expansion, using the geometry of the gas gap switch shown in FIG. 1. According to this prior art attempt, the hot side member 101 and the cold side member 103 are constructed of different materials which have different coefficients of thermal expansion, C.sub.TE. The hot side member 101 is chosen to have a coefficient of thermal expansion which is much smaller than the coefficient of thermal expansion of the material used for the cold side member 103. As the temperature of the switch elements 103 and 101 rise, the dimensions of the cold side element 103 would expand at a faster rate than the dimensions of the hot side element 101. Thus, as temperature rises, a gap would open up between 103 and 101. This would cause the transition of the switch from the ON state to the OFF state. In order to reverse the state from OFF back to ON, the cold side 103 would need to be cooled down, thereby shrinking its dimensions so as to come into contact with the hot side element 101.
The reason that this prior art attempt at a differential coefficient of thermal expansion type of thermal switch failed is because it is impossible to solve the problem of being able to make the surfaces of the two elements 101 and 103 meet together in a good thermal conducting relationship with good contact on a reliable basis. The problem goes beyond merely the challenges of how to machine and polish the surface of the frustoconical member 106 to sufficiently match the shape of the surface of the frustoconical cavity 104. The primary problem is how to align the member 106 and the cavity 104 so that they reliably contact one another so as to produce positive engagement across substantially all of their opposed surface area. The smallest misalignment results in only point contacts at a few places, which is unacceptable because such minimal contact points produce very poor thermal conductivity for the ON state. Because of this mechanical mismatch of the surfaces, the two surfaces cannot reliably mate to one another and provide a reliable ON state. In addition, the proper alignment of the frustoconical member 106 and frustoconical cavity 104 is not easily verified when the switch is assembled. Consequently, the gap 105 is difficult to maintain and the switch does not work properly in the OFF condition.
Accordingly, what is needed is a cryogenic thermal switch that provides an adequate on-off ratio and a reliable ON state conductance. What is also needed is a cryogenic thermal switch which has a simple construction and which operates reliably.